Generally, when a web browser requests information, for example a web site or file, the request is filtered through the browser's cache system. The cache system will determine whether the file is stored locally. If the file is not stored locally, the request may be forwarded, for example, to a web server.
Web applications, such as web sites, may regularly update HTML, javascript, CSS, and other auxiliary files. Some systems, such as Google Maps update javascript files on a regular basis. These files may be hundreds of kilobytes in size, and on average may take several seconds to transmit to the browser. Each time these files are updated, the web browser must request them in full from the web server.
Transmitting these auxiliary files may add significant latency to a web application's startup time, and may increase the amount of traffic sent across the network.